OBJECTIVETo investigate the clinical significances and formation mechanism by analyzing the characteristics of the reverse phase nystagmus parameters from benign paroxysmal positional vertigo (BPPV) positioning test.

METHODSThere were 183 cases with BPPV, including 108 cases (59.0%) of posterior semicircular canal canalithiasis, 55 cases (30.1%) of horizontal semicircular canal canalithiasis, and 15 cases (8.2%) of horizontal and posterior semicircular canal cupulolithiasis and 5 cases (2.7%) of anterior semicircular canalithiasis . The video-nystagmography was utilized in positioning tests to induce nystagmus. The direction, intensity, time parameters characteristic of vertical nystagmus in Dix-Hallpike test and horizontal nystagmus in roll test were analyzed and compared.

RESULTSThere were no reversal phase nystagmus in 15 cases of semicircular canal cupulolithiasis and 5 cases of anterior semicircular canalithiasis. After the disappearance of vertical nystagmus which induced by hanging position (the first phase nystagmus) in 108 cases of posterior semicircular canalithiasis of Dix-Hallpike test, there was 54 cases(50.0%) of posterior semicircular canal canalithiasis displayed downward vertical nystagmus (reverse phase nystagmus) . The latency, duration time and intensity of the first phase nystagmus and reverse phase nystagmus were [(2.00 ± 1.10) s, (3.54 ± 1.42) s], [ (16.27 ± 4.95) s, (61.65 ± 33.69)s] and [ (51.80 ± 25.25) °/s, (10.65 ± 6.29)°/s] respectively; 43 cases(78.2%) of horizontal semicircular canal canalithiasis displayed the opposite to turning head (reverse phase nystagmus) after the horizontal nystagmus, similar with turning head disappeared in Roll test. The latency, duration time and intensity of the first phase of nystagmus and reverse phase nystagmus were [ (1.67 ± 1.07) s, (3.57 ± 1.89)s], [ (25.19 ± 9.74) s, (70.48 ± 40.26)s] and [ (68.47 ± 30.18) °/s, (11.22 ± 8.78)°/s] respectively. Comparing with the latency, duration time, intensity of first phase nystagmus and reverse phase nystagmus of posterior and horizontal semicircular canal canalithiasis, the differences had statistical significances (P < 0.05). Comparing with the first phase nystagmus of reverse phase and no reverse phase nystagmus canalithiasis, the difference of nystagmus intensity had statistical significances (P < 0.05); but the differences of latency and duration of nystagmus had no significant difference (P > 0.05).

CONCLUSIONSIt is common in PSC-Can and HSC-Can patients that reverse phase nystagmus is one of the clinical features of canalithiasis. It appears in side head position of Rolling test or the hanging of Dix- Hallpike test. More power of the first phase nystagmus has, reverse phase nystagmus will be induced much easier. In comparison of the reverse phase nystagmus, the first phase nystagmus has the shorter incubation and duration, but it has more power. It is helpful to avoid interruptions of clinical statolith positioning and reset since we recognize the reverse phase nystagmus. The mechanism might be similar to the vestibular mirror image nystagmus. It is another form of the vestibular mirror image nystagmus in BPPV patients.